Abstract: The red bedded mudstone, known for its expansive and water-sensitive nature, poses a long-term potential threat to the construction and post-construction deformation control of regional highways and high-speed railroads. In this study, an in-situ water infiltration response characteristics test was conducted on slightly-expansive mudstone foundations using a red mudstone road cut in Lanzhou, Gansu Province. The test aimed to analyze the spatial and temporal evolution of swelling deformation volume, swelling force, and volumetric water content of the mudstone foundations, as well as to compare the differences between laboratory tests and in-situ tests. The results reveal that the water infiltration forms in the red mudstone foundation include fissure flow and pore flow. The distribution of water in the rock mass exhibits significant spatial and temporal heterogeneity, with rock mass fissures promoting seepage and expansion. During the infiltration process, the water absorption and expansion of the mudstone exhibit significant time dependence. The expansion amount and expansion force of the in-situ foundation experience stages of rapid increase, slow growth, and eventually fail to converge. After reaching the infiltration peak, the surface of the mudstone gradually softens or even turns into mud, leading to a decrease in the bearing capacity of foundation. Additionally, through the analysis of macroscopic expansion time history variation characteristics and microscopic pore structure distribution laws of indoor and in-situ mudstone, it is found that laboratory tests provide limited characterization of the water infiltration response characteristics of in-situ soil.

Key words: rock mechanics, red mudstone, slightly-expansive, in-situ test, water infiltration